In accordance with high integration of semiconductor devices in recent years, requirements for flattening of semiconductor silicon wafers used therefore have been rigorous more and more. Moreover, in order to increase the yield of semiconductor chips, flatness up to a region in the vicinity of edges of a wafer is required.
A desired shape of the silicon wafer is determined by mirror-polishing process which is the final process. Particularly, in the case of silicon wafers having a large diameter, e.g., a diameter of 300 mm, etc., primary polishing at double-side polishing process is performed in order to satisfy the specification of rigorous flatness thereafter to perform surface-secondary polishing process and the finish polishing process on one side surface for the purpose of improving scratches of the surface and/or surface roughness. In the surface-secondary polishing process and the finish polishing process on one side, it is required to maintain flatness prepared by the double-side primary polishing process, and to finish into a perfect mirror-surface free from defects such as scratches, etc. on the surface side.
The schematic view of a typical single-side polishing apparatus is illustrated in FIG. 5. This single-side polishing apparatus 101 comprises a turn table 106 on which a polishing pad 107 is stuck, polishing-agent-supplying mechanism (not shown), and a polishing head 120, etc. In such a single-side polishing apparatus 101, a workpiece W is held by means of the polishing head 120 to deliver polishing agent onto the polishing pad 107 from the polishing-agent-supplying mechanism, and to respectively rotate the turn table 106 and the polishing head 120 to allow the surface of the workpiece W to slidably come into contact with the polishing pad 107 to thereby perform polishing.
Examples of a method of holding a workpiece by a polishing head include a method of sticking the workpiece through an adhesive, such as wax, on a flat disc-shaped plate. The other methods include a method of holding a workpiece through an elastic film, called a backing film, adhered on a workpiece holding plate, for the purpose of suppressing an influence of transference of unevenness of a polishing head body and the workpiece holding plate, and a method of pressing a workpiece against a polishing pad by supplying pressurized fluid, such as air, to the back surface of a rubber film, as a workpiece holding member, to inflate the rubber film with an uniform pressure, which is called a rubber-chuck method (See Patent Document 1, for example). Moreover, for the purpose of suppressing sags at an outer circumferential part to improve flatness, there is also proposed a polishing head in which a retainer ring for pressing a polishing pad is disposed on the outside of a workpiece.
An example of the configuration of a conventional polishing head by the rubber-chuck method is illustrated in FIG. 5. The polishing head 120 illustrated in FIG. 5 has a structure, so-called the rubber-chuck structure, in which a rubber film (rubber material) 122 is attached so as to cover at least the lower-surface part and the side-surface part of a disc-shaped mid plate 125, and fluid is supplied to the back surface of the rubber film to press a workpiece W.
An annular guide ring 123 for holding the side surface of the workpiece W during polishing is connected to a polishing head body 121. The polishing head is configured to move the height position of the guide ring 123 vertically with a height-adjusting mechanism 124 connected to the polishing head body 121. The workpiece is stably held and polished while maintaining a constant distance between the polishing pad and the guide ring 123 connected to the polishing head body 121.
As described above, the polishing head presses a workpiece by inflating the rubber film of the polishing head while holding the polishing head at a fixed height to maintain a constant distance between the polishing head and the polishing pad. A polishing head, represented by the above polishing head, using the method of maintain a constant distance between the polishing head and the polishing pad needs to adjust the height position of the polishing head from the polishing pad to the same position in every polishing.
In the case where this height position is not suitably adjusted, in the instance where the flatness of a workpiece to be polished may be degraded, and/or positions every polishing are not uniform, allowance in machining between workpieces to be polished may not become uniform so that unevenness of flatnesses takes place. In the case where such adjustment is poor, when spacing is too narrow, contact between the polishing pad and the guide ring may take place so that the polishing pad is damaged. On the contrary, when the spacing is too broad, there is the risk that wafer may be flied out from a gap between the polishing pad and the guide ring.
A conventional method for adjusting a height position of a polishing head, as illustrated in FIG. 5, involves measuring a distance from the polishing head 120 by using a laser displacement meter 130, disposed near the mounting face of the polishing head 120, and adjusting the height of the polishing head on the basis of the measured distance such that the polishing head keeps a desired distance from the polishing pad 107.